The present invention relates to a power semiconductor module for use in a power conversion apparatus, such as an inverter or the like, and in particular, it relates to a highly reliable module structure of an insulated gate bipolar transistor (IGBT) module.
A conventional IGBT module structure is shown in FIG. 2. As seen in FIG. 2, a ceramics substrate 25 is soldered on a Cu base 11, and the substrate 25 has a Si chip 16 (i.e., IGBT, free-wheeling diode (FWD)) mounted thereon. An electric connection between a main terminal 12 or a control terminal (not shown) with the Si chip 16 is provided by soldering the above-mentioned terminals to the ceramics substrate 25. These terminals are embedded in a cap of the module, which is made of resin and is depicted as terminal block 20, which is located in the upper portion of the module.
In JP-A No. 7-263621, there is disclosed a module structure using a so-called terminal-inserted case 35, where main terminal 12 and control terminal 30 are inserted in the case of the module, as shown in FIG. 3. In this module structure, Si chips 16, including an IGBT and a FWD, are soldered on a copper plate for reducing the thermal resistance (i.e., heat spreader 34), and the heat spreader 34 is soldered on a surface of an A1 metal core printed wiring board 33, which is prepared by coating a resin 32 on a surface of an A1 substrate for electrically insulating the chips from the substrate. Then, the A1 metal core printed wiring board 33, which supports the thermal spreader 34, IGBT, FWD and the like, is bonded to a terminal-inserted case 35 using a thermosetting silicone adhesive 31. For this module structure, using the terminal-inserted case 35 of FIG. 3, there are two ways for electrically connecting its terminals with the chips. One is by wire bonding alone, as shown in FIG. 3, and the other is by soldering the terminals to the ceramics substrate.
With reference to FIG. 4, JP-A No. 6-224314 discloses another semiconductor device prepared by encasing a metal base 9 in a module case 41 in such a manner that the metal base 9 and external input and/or output terminals 40 are molded so as to be integral with the module case 41. In FIG. 4, electronics components 42 are soldered on a surface of a thick film circuit board 43, and input and/or output terminal 44 on the surface of the thick film circuit board 43 is electrically connected to an external connection terminal using a metal wire 19.
The prior art semiconductor devices referred to above have the following inherent problems which deteriorate the reliability of the module.
In the structure of FIG. 2 using the terminal block 20, main terminal 12, which is soldered to a copper foil on the surface of the ceramics substrate 25, has a substantially vertical orientation. Since there exists a large difference in the coefficient of linear expansion between main terminal 12 made of a metal and that of the ceramics substrate 25, the solder 23 connecting the ceramics substrate 25 and the main terminal 12 is subjected to a large stress due to a repeated heat cycle of heating and cooling of the module. The mechanical strength of the solder 23 deteriorates due to application of this stress, consequently allowing a crack to propagate therein and thereby causing a failure of the connection. In order to reduce the L occurrence of such an adverse phenomenon, a bending portion 24 is provided in the main terminal 12, as shown in FIG. 2, so as to absorb the stress. However, the provision of this bending portion 24 results in an increase in the length of the main terminal 12, thereby substantially increasing its inductance, with the result that the performance of the module is deteriorated. Namely, according to the module structure of FIG. 2, it is difficult to achieve a high reliability and a high performance for the module at the same time.
FIG. 3 illustrates a module structure using the terminals-inserted case 35 in which electrical connection is provided solely by wire bonding. The advantages of using the terminals-inserted case reside in the fact that it can eliminate the terminal block 20 of FIG. 2, thereby reducing its manufacturing cost, and it can avoid deterioration of the reliability of the solder due to the heat cycle described above at the same time. However, the bonding of A1 metal core printed wiring board 33 with the terminals-inserted case 35 must be done using a thermoset adhesive because a room temperature setting adhesive, which has a relatively low reliability, cannot be used in a stringent environment subject to a heat cycle of heating and cooling where IGBT modules are used. Further, even though a highly reliable room temperature adhesive may be developed in the future, the problem associated with degassing of the adhesive during the process of curing the encapsulating gel or resin at high temperatures, which is inevitable to a power semiconductor manufacture, will still remain, thereby causing an obstruction to the curing of gel 18.
Use of a thermosetting silicone resin adhesive 31 for bonding the terminals-inserted case 35 and the A1 metal core printed wiring board 33 has resulted in a highly reliable bonding therebetween. However, since wire bonding must be done after completion of bonding between the inserted-case 35 and the A1 metal core printed wiring board 33, there arises a problem in that an electrode pad for wire bonding is contaminated by silicone oil and the like which is produced at the time of the curing of the silicone resin adhesive 31. This contamination reduces the adhesive strength of the wire bonding, thereby decreasing the reliability of the module. Namely, the module construction of FIG. 3, although it eliminates the problem of deterioration of the solder 23 for connecting to the terminal of FIG. 2, is associated essentially with another problem in that the bonding strength between the wire and the pad is lowered due to contamination, thereby lowering the reliability of the module. This problem occurs irrespective of whether the wire bonding is carried out by an ultrasonic bonding method, a thermo-compression method or a combined ultrasonic thermo-compression method.
In the case of the prior art arrangement of FIG. 4 in which the metal base 9 is molded integral with the terminals-inserted case 41, the above-mentioned two problems of the deterioration of the solder and the contamination of the bonding pad due to use of the thermosetting resin adhesive have been solved. However, when the module construction of FIG. 4 is applied to a power semiconductor module which requires a withstand voltage over several hundred volts and allows a large current to flow therethrough, it becomes essential to encapsulate the module with an encapsulant, such as a gel, epoxy resin or the like, to fasten the terminal connected to its power circuit to the module with a screw so as to ensure the reliability of the connection, and to ensure that the above-mentioned screw will have a sufficient insulation and distance from the surrounding electrical elements to ensure the reliability of the module. However, according to the prior art structure disclosed in FIG. 4, there are still problems, such as gel which is poured as an encapsulant leaking through a peripheral portion of the module where the metal base is inserted into the module case and an external input and/or output terminal 40 not being able to be fastened sufficiently with a screw.